getting current time as double

linux, mac :

#include <sys/time.h>

static double now_ms(void) 
{ 
struct timeval tv; 
gettimeofday(&tv, NULL); 
return tv.tv_sec*1000. + tv.tv_usec/1000.; 
}

windows

static double now_ms(void) 
{ 
static LARGE_INTEGER frequency;
static bool dummy = QueryPerformanceFrequency(&frequency);

LARGE_INTEGER count;
QueryPerformanceCounter(&count);
if(frequency.QuadPart == 0) return 0;

return (double)count.QuadPart/(double)frequency.QuadPart;
}